A ScAlMgO4 substrate has been used as a substrate for epitaxial growth of a nitride semiconductor, such as GaN (see, for example, Patent Literature 1). FIG. 5 is a diagram showing an example of the method for producing an ordinary ScAlMgO4 substrate described in Patent Literature 1. As shown in FIG. 5, the ordinary ScAlMgO4 substrate has been produced by cleaving an ScAlMgO4 bulk material.
Patent Literature 1: JP-A-2015-178448
As shown in Patent Literature 1, ScAlMgO4 having cleavage property can be easily formed into a substrate by utilizing the cleavage property thereof, but the cleavage property may affect the robustness of the substrate. For example, in the production of a wafer or a device from an ScAlMgO4 substrate, chipping and cracking may occur at the edge portion of the substrate in some cases, which can affect the yield and the like. Under these circumstances, there has been a demand of a substrate having high robustness.